Silicon Condenser Microphones

Silicon (MEMS) condenser microphones

Fig. 1: Silicon wafer with MEMS condenser microphones
Fig. 1: Silicon wafer with MEMS condenser microphones

Silicon microphones are made on silicon wafers with the methods of micromachining. They are often referred to as “MEMS” microphones, derived from the term “Micro Electro Mechanical System”. The fabrication methods are borrowed from microelectronics and consist of lithography-, doping-, deposition- and etching processes. A large number of microphones with very reproducible properties can be produced on a single wafer, as Fig. 1 shows.

Fig. 2: Schematic design of a single-chip MEMS condenser microphone
Fig. 2: Schematic design of a single-chip MEMS condenser microphone

While several microphone types, such as piezoelectric or FET-modulating transducers, may be implemented in silicon [1], the most successful variety is the capacitive silicon microphone. This consists of a membrane separated by an air gap from a rigid back electrode. If the system is polarized with a small dc-bias, excitation of membrane vibrations by a sound wave generates an electric output signal proportional to the sound pressure.

Such silicon condenser microphones were first proposed in 1983 [2] and implemented as two-chip sensors, consisting of a membrane chip and a backplate chip, in 1985 [3]. They were later made as single-chip microphones, using a sacrificial-layer technology where the air gap is obtained by removing an oxide layer originally deposited between membrane and back plate, by an etching process [2,4]. A typical single-chip microphone is shown in Fig. 2 [5].

Typical silicon microphones have membrane areas of about 1 mm2, membrane thicknesses of 0.2 to 0.4 µm, resonance frequencies in the near ultrasonic range and sensitivities of approximately 10 mV/Pa. They are furthermore shock resistant, insensitive to vibration, and may be operated permanently at temperatures up to 100°C and, for short periods of time, up to 260°C [6,7]. Thus, they can be produced as SMD devices for printed circuit boards. Silicon microphones have been commercially produced since 2002 and are being used primarily in smartphones where often two to four units are being employed for improvement of the S/N ratio but also for directivity gains. They are being produced in numbers of several billion annually.

Literature

  • [1] G. M. Sessler, J. Audio Engineer. Soc. 44, 16-22 (1996).
  • [2] D. Hohm and G. M. Sessler, in Proc. of the 11th Int. Congress on Acoustics (Paris, 1983), Vol. 6, pp. 29–32.
  • [3] D. Hohm, in Fortschritte der Akustik – DAGA 1985, pp. 847-850; D. Hohm and G. Hess, J. Acoust. Soc. Am. 85, 476-480 (1989).
  • [4] P. R. Scheeper, A. G. H. van der Donk, W. Olthuis, and P. Bergveld, J. Microelectromech. Systems 1, 147-154 (1992).
  • [5] C. Thielemann and G. M. Sessler, Acustica-acta acustica 83, 715-720 (1997).
  • [6] G. W. Elko and K. P. Harney, Acoustics Today, 5 (issue 2), 4-13 (2009).
  • [7] J. Czarny, HAL archives-ouvertes (tel-01247487) (2015).